Method for producing isocynates

ABSTRACT

The invention relates to a process for the production of isocyanates by reaction of primary amines with phosgene, in which isocyanate is used as solvent, wherein some or all of isocyanate used as solvent is added to the reaction mixture only after the amine and phosgene have been physically combined.

DESCRIPTION

The present invention relates to a process for the production ofisocyanates by reaction of amines with phosgene using isocyanate assolvent, the isocyanate used as solvent being added to the reactionmixture only after the amine and phosgene have been physically combined.

Various processes for the production of isocyanates by reaction ofamines with phosgene in the presence of isocyanate are known.

DE-A 1,192,641 describes a process for the production of isocyanates byreaction of amines with phosgene, the isocyanate produced during thereaction being used as solvent for phosgene.

DE-A 2,252,068 describes a process for the production of organicisocyanates, wherein amine is caused to react with preheated phosgene inthe presence of an excess of isocyanate at temperatures and pressuresselected so as to give a homogeneous liquid phase.

U.S. Pat. No. 2,822,373 describes a continuous process for theproduction of isocyanates, in which a phosgene solution is mixed with asolution of an organic amine in a turbulent reactor circulation system.In this case, the fresh phosgene solution is combined with thecirculated reaction solution prior to blending with the amine solution.

WO 96/16028 describes a continuous process for the production ofisocyanates by reaction of corresponding primary amines with phosgene inthe presence of an isocyanate acting as solvent, wherein the amine iscaused to react with phosgene, which is dissolved in the isocyanate in aconcentration of from 10 to 60 wt %.

It is also known that the use of a high phosgene excess over the aminogroups used leads to high selectivities toward the isocyanate producedand can thus have a decisive influence on the economical value of theprocess. However, with an increasing ratio of phosgene to amino groupsthere is a rise in the phosgene holdup of the plant, which should,however, be minimized on account of the toxicity of phosgene.

It is thus an object of the present invention to provide a process forthe production of isocyanates which makes it possible to carry out thereaction with high selectivity without raising the phosgene hold-up orlowering the space-time yield.

The object of the invention is achieved, unexpectedly, in the productionof isocyanates by the reaction of amine with phosgene, by adding anisocyanate as solvent, which solvent is added not, as described in theprior art, to the phosgene prior to the reaction, but to the reactionmixture formed by physically combining the phosgene and amine.

Thus the present invention relates to a process for the production ofisocyanates by reaction of primary amines with phosgene, in whichisocyanate is used as solvent, wherein some or all of the isocyanateused as solvent is added to the reactants only when the amine andphosgene have been physically combined.

The process of the invention includes continuous, semicontinuous, andbatch processes. Preference is given to continuous processes. During theproduction of isocyanate by reaction of a primary amine with phosgenethere is formed, in an initial fast step, α-cording to followingreaction scheme, the intermediate carbamoyl chloride, which decomposesin the rate-determining, slow step in a balanced reaction to formisocyanate and HCl.

R—NH₂+COCl₂→R—NH—COCl+HCl⇄R—NCO+2 HCl,

in which R is an organic radical.

Furthermore the resulting hydrogen chloride can react with amines toform amine hydrochlorides.

The essential feature of the process of the invention is that theaddition of the isocyanate acting as solvent does not occur until theamine and phosgene have been physically combined, preferably by mixing.In one embodiment of the invention, the solvent is added at a time orpoint at which at least 50%, preferably at least 80%, more preferably atleast 90%, and most preferably at least 95% of the amino groups usedhave already undergone reaction.

The rate of the reaction between phosgene and amine or between hydrogenchloride and amine is primarily governed by the type of isocyanate to besynthesized and the reaction temperature used. Correspondingly, theaddition of isocyanate acting as solvent can take place some timebetween 0.1 milliseconds and 10 minutes following physical combinationof phosgene and amine.

In the process of the invention, some or all of the isocyanate used assolvent is added to the reaction mixture, ie the reactants amine andphosgene, only after the amine and phosgene have been physicallycombined. In a preferred embodiment, the portion added to the reactionmixture after the amine and phosgene have been physically combined is atleast 25 wt %, preferably at least 50 wt %, more preferably at least 75wt %, and most preferably at least 90 wt %, based on the total amount ofisocyanate to be used as solvent. The portion thereof which is not to beadded after the amine and phosgene have been physically combined, can beadded prior to the reaction either to the amine or, preferably, to thephosgene.

In the process of the invention, use can be made of any one primaryamine or a mixture of two or more such amines. Preference is given toaromatic amines, particularly those in the diaminodiphenylmethane seriesor their higher homologs. Examples thereof are methylenediphenylamine(MDA; individual isomers, mixture of isomers and/or oligomers thereof),toluylenediamine (TDA), n-pentylamine, 6-methyl-2-aminoheptane,cyclopentylamine, R,S 1-phenylethylamine, 1-methyl-3-phenylpropylamine,2,6-xylidine, 2-(N,N-di-methylamino)ethylamine.2-(N,N-diisopropylamino)ethylamine, C11-neodiamine, isophoronediamine,3,3′-diaminodiphenylsulfone, and 4-aminomethyl-1,8-octanediamine. MDAand TDA are preferably used.

The process of the invention is thus suitable for use in the synthesisof any desired isocyanates. The process can be used with particularadvantage for the production of methylene (diphenyldiisocyanate) (MDI)and toluylene-diisocyanate (TDI).

The isocyanate used as solvent is preferably the isocyanate to besynthesized. It may come from an external source or, preferably, betaken from the process of the invention and recycled. Alternativelyhowever, other suitable isocyanates, or mixtures thereof, may be used assolvent.

An additional inert solvent can be co-used in the process of theinvention. This additional inert solvent is usually an organic solventor a mixture thereof. Chlorobenzene, dichlorobenzene, trichlorobenzene,toluene, hexane, diethyl isophthalate (DEIP), tetrahydrofuran (THF),dimethylformamide (DMF), benzene, and mixtures thereof are preferred.Particular preference is given to chlorobenzene. The additional inertsolvent can be added to the amine preferably at the commencement of thereaction. The inert solvent is usually employed in a concentration offrom 5 to 1000 wt %, and preferably from 50 to 500 wt %, based on theamount of amine used.

The process of the invention will now be described in detail withreference to a general flow sheet illustrating a continuous process, asshown in FIG. 1. The elements depicted in FIG. 1 are as follows:

I phosgene receiver

II amine receiver

III first mixing device

IV second mixing device

V reaction device

VI first separating device

VII second separating device

VIII isocyanate feedstock

IX phosgene purifier

X solvent purifier

1 phosgene feedline

2 amine feedline

3 inert solvent feedline

4 separated hydrogen chloride and inert solvent

5 recycled isocyanate stream

6 discharged hydrogen chloride

7 separated isocyanate

8, 11 separated inert solvent

9 purified inert solvent

10 purified phosgene

The amine from amine receiver II and phosgene from phosgene receiver Iare mixed in a suitable first mixing device III. The mixture of amineand phosgene is mixed, in a second mixing device IV, with isocyanateacting as solvent and passed to the reaction device V. Suitable mixingdevices are, for example, nozzles or blender reactors. It is alsopossible to carry out the two mixing operations of mixing devices IIIand IV in a common mixing device but in discrete regions, as explainedbelow with reference to the preferred embodiment illustrated in FIG. 2.

After mixing, the mixture is passed to a reaction device V. Suitablereaction devices are, for example, tubular reactors, tower reactors,reaction vessels or reaction columns. Tubular reactors are preferred.Also useful are contrivances which are both mixer and reactor, forexample, tubular reactors having flanged-on nozzles.

The two separating devices VI and VII are preferably distillation units.In the first separating device VI, hydrogen chloride and, optionally,inert solvent and/or small portions of the isocyanate stream are usuallyseparated from the isocyanate stream. In the second separating deviceVII, preferably inert solvent is separated and then purified (X) andrecycled to amine receiver II.

A preferred embodiment of the process of the invention involves, asillustrated in FIG. 2, the use of a mixing device described below, whichis directly followed by a reaction device.

FIG. 2 illustrates a preferred assembly for the process of theinvention. The elements depicted in FIG. 2 are as follows:

1 amine feed

2 phosgene feed

3 isocyanate feed

4 a injector tube

4 b injector tube

5 diffuser

6 tubular reactor

7 axis

8 annular gap in the phosgene feedline

9 annular gap in the isocyanate feedline

In continuous phosgenation, the amine or the amine solution 1 isinjected along the axis 7 into the mixing device, usually at rates offrom 5 to 60 m/s. The phosgene or phosgene solution passes into themixing device through an annular gap in the phosgene feedline 8 at ratesalso ranging from 5 to 60 m/s. The two streams of amine and phosgene arephysically combined (corresponding to the first mixing device in FIG. 1)and passed through an optional injector tube 4 a, after which isocyanateis fed in through annular gap 9 in the isocyanate feedline(corresponding to the second mixing device in FIG. 1). After passingthrough an optional injector tube 4 b, the reaction mixture passesthrough diffuser 5 into tubular reactor 6. Following a residence time offrom 10 s to 20 min, the resulting crude isocyanate solution is removedfrom the tubular reactor.

In a preferred embodiment, the injector mixing device used is an axiallysymmetrical injector tube device having an axial amine feed and phosgeneand isocyanate feeds effected via two off-axis annular gaps.

The optimal temperature range for the process of the invention isgoverned, inter alia, by the type and concentration of the solvent andby the isocyanate to be synthesized. Generally, the temperature in themixing unit is between −20° C. and 300° C., preferably between 10° C.and 200° C., and more preferably between 80° C. and 150° C. Thetemperature in the reactor is generally between 10° C. and 360° C.,preferably between 40° C. and 210° C., and more preferably between 100°C. and 180° C. Furthermore, the absolute pressure is generally between0.2 bar and 50 bar, preferably between 1 bar and 25 bar, and morepreferably between 3 and 17 bar.

The residence time of the fluid in the mixing device and the reactor is,in all, between 12 s and 20 min, preferably in the range of from 36 s to16 min, and more preferably between 60 s and 12 min.

The molar ratio of phosgene used to amino groups is from 1:1 to 12:1,preferably from 1.1:1 to 4:1. Amine and phosgene can be used free fromsolvent or dissolved in one or more of the aforementioned inertsolvents. Alternatively, the phosgene can be injected as a gas into theamine solution. In addition, the phosgene may be premixed with a portionof the isocyanate used as solvent, as described above.

The amount of isocyanate used as solvent in the process of the inventionis generally from 10 to 1000 wt %, preferably from 50 to 500 wt %, andmore preferably from 100 to 400 wt %, based on the amount of phosgeneused.

Following the reaction, the mixture of substances is separated intoisocyanate, solvent, phosgene, and hydrogen chloride, preferably bymeans of rectification. Small amounts of by-products remaining in theisocyanate can be separated from the desired isocyanate by means ofadditional rectification or, alternatively, by crystallization.

Depending on the reaction conditions chosen, the crude end product maycontain inert solvent, carbamoyl chloride, and/or phosgene and can befurther processed by known methods (cf, eg, WO 99/40059). Furthermore,it may be advantageous to pass the product over a heat exchanger afterdischarge.

The invention is illustrated below by the following examples.

EXAMPLE 1

90 g of a toluylenediamine mixture (TDA mixture), comprising 80 wt %2,4-TDA and 20 wt % 2,6-TDA was dissolved in 360 g of monochlorobenzene(MCB). The resulting TDA solution was injected into the mixing deviceillustrated in FIG. 2 along the axis at a rate of 30 m/s and athroughput of 1.8 L/h, at a temperature of 50° C. Simultaneously, 1.6 kgof a 25% strength phosgene solution, which had been produced from 400 gof phosgene and 1.2 kg of MCB, were injected at a temperature of 30° C.through the first annular gap of the nozzle mixer at a rate of 30 m/sand at an angle of 45° to the axis. Then 1.2 kg oftoluylene-diisocyanate, comprising 80 wt % 2,4-TDI and 20 wt % 2,6-TDI,were injected at a temperature of 30° C. into the axial injector tubethrough the second annular gap at an angle of 90° to the axis of themixing device and at a rate of 25 m/s in the plane of entry. The mixturepassed directly from the nozzle mixer into a tubular reactor, which hada capacity of ca 300 mL and a length of 24 m and provided a residencetime for the total mixture of ca 1.4 min and was kept at a temperatureof from ca 130° to 140° C. Before and after synthesis, the apparatus wascontinuously rinsed with monochlorobenzene as inert solvent. Followingremoval, by distillation, of the phosgene and chlorobenzene, TDI wasisolated at a purity of ca 99.2% (GC) and a yield of 99.1%. The phosgenehold-up in the reactor was, based on the incoming phosgene mass flow,not more than ca 38 g. This value was calculated on the assumption thatphosgene was not consumed during the reaction. This gave a phosgenehold-up of ca 76 g for a production output of 1 kg of TDI per hour.

COMPARATIVE EXAMPLE 2 USING AN ISOCYANATE/PHOSGENE SOLUTION

In a manner similar to that described in Example 1, 90 g of atoluylenediamine mixture (TDA mixture) comprising 80 wt % 2,4-TDA and 20wt % 2,6-TDA were dissolved in 360 g of monochlorobenzene. The resultingTDA solution was injected into the mixing device illustrated in FIG. 2along the axis at a rate of 30 m/s and a throughput of 1.8 L/h, at atemperature of 50° C. Simultaneously, 1.6 kg of a 25% strength phosgenesolution, which had been produced from 400 g of phosgene and 1.2 kg oftoluylene diisocyanate, comprising 80 wt % 2,4-TDI and 20 wt % 2,6-TDI,were injected at a temperature of 30° C. through the first annular gapof the nozzle mixer at a rate of 30 m/s and at an angle of 45° to theaxis. The mixture passed directly from the nozzle mixer into a tubularreactor, which had a capacity of ca 180 mL and a length of 14.4 m andprovided a residence time for the total mixture of ca 1.4 min and waskept at a temperature of from ca 130° to 140° C. Before and aftersynthesis, the apparatus was continuously rinsed with monochlorobenzeneas inert solvent. Following removal, by distillation, of the phosgeneand chlorobenzene, TDI was isolated at a purity of ca 99.1% (GC) and ayield of 98.5%. The phosgene hold-up in the reactor was, based on theincoming phosgene mass flow, not more than ca 38 g. This value wascalculated on the assumption that phosgene was not consumed during thereaction. This gave a phosgene hold-up of ca 76 g for a productionoutput of 1 kg of TDI per hour.

COMPARATIVE EXAMPLE 3 USING A MCB-PHOSGENE SOLUTION

In a manner similar to that described in Example 1, 90 g of atoluylenediamine mixture (TDA mixture) comprising 80 wt % 2,4-TDA and 20wt % 2,6-TDA were dissolved in 360 g of monochlorobenzene. The resultingTDA solution was injected into the mixing device illustrated in FIG. 2along the axis at a rate of 30 m/s and a throughput of 1.8 L/h, at atemperature of 50° C. Simultaneously, 1.6 kg of a 25% strength phosgenesolution, which had been produced from 400 g of phosgene and 1.2 kg ofMCB, were injected at a temperature of 30° C. through the first annulargap of the nozzle mixer at a rate of 30 m/s and at an angle of 45° tothe axis. The mixture passed directly from the nozzle mixer into atubular reactor, which had a capacity of ca 375 mL and a length of 30 mand provided a residence time for the total mixture of ca 2.7 min andwas kept at a temperature of from ca 130° to 140° C. Before and aftersynthesis, the apparatus was continuously rinsed with monochlorobenzeneas inert solvent. Following removal, by distillation, of the phosgeneand chlorobenzene, TDI was isolated at a purity of ca 99.1% (GC) and ayield of 97.4%. The phosgene hold-up in the reactor was, based on theincoming phosgene mass flow, not more than ca 72 g. This value wascalculated on the assumption that phosgene was not consumed during thereaction. This gave a phosgene hold-up of ca 147 g for a productionoutput of 1 kg of TDI per hour.

SUMMARY OF THE EXAMPLES

Table 1 below compares the results of Example 1 and Comparative Examples2 and 3. Table 1 clearly shows that the process of the invention leadsto a rise in selectivity without raising the phosgene hold-up orlowering the space-time yield.

Comparative Comparative Example Example 1 Example 2 Example 3 Phosgenehold-up for a 76 g 76 g 147 g production output of 1 kg/h Yield 99.2%98.5% 97.4   Distillation purity 99.1% 99.1% 99.1%

What is claimed is:
 1. A process for the production of isocyanates byreaction of primary amines with phosgene, in which isocyanate is used assolvent, wherein some or all of the isocyanate used as solvent is addedto the reactants only after the amine and phosgene have been physicallycombined.
 2. A process as defined in claim 1, which is carried outcontinuously.
 3. A process as defined in claim 1, wherein portions ofthe isocyanate produced are recycled and used as solvent.
 4. A processas defined in claim 1, wherein the amount of isocyanate added to thereaction mixture only after the amine and phosgene have been physicallycombined is at least 25 wt %, based on the total amount of isocyanateused as solvent.
 5. A process as defined in claim 1, wherein the amineused is methylenedi(phenylamine) or toluylenediamine.
 6. A process asdefined in claim 1, wherein the reaction of amine with phosgene iscarried out in a tubular reactor having an upstream injector mixingdevice.
 7. A process as defined in claim 6, wherein the injector mixingdevice used is an axially symmetrical injector tube device having anaxial amine feed and phosgene and isocyanate feeds effected through twooff-axis annular gaps.